As this kind of valve gear, there has been known a valve gear which opens and closes the intake valve by rotating a cam shaft of the internal combustion engine by a stepping motor (Japanese Patent Application Laid-Open (JP-A) No. 8-177536). In addition, there is JP-A No. 59-68509 as a prior technical document relevant to the present invention.
A cam shaft torque caused by a valve spring and inertia is applied to the cam shaft as a resistance against the rotation. However, the cam shaft torque is fluctuated in correspondence to a rotation number (a rotating speed) of the engine, and there is a possibility that a rotation region in which a desired valve gear characteristic cannot be obtained is generated due to the fluctuation.